System and method of processing memory requests in a pipelined memory controller

ABSTRACT

A method for processing multiple memory requests in a pipeline. Each memory request is processed in part by a plurality of stages. In a first stage, the memory request is decoded. In a second stage, the address information for the memory request is processed. In a third stage, the data for the memory request is transferred. A request buffer is used to hold each of the memory requests during the processing of each of the memory requests.

RELATED APPLICATIONS

[0001] This application is a continuation application of U.S. patentapplication Ser. No. 09/127,282, titled “Method of Processing MemoryRequests In A Pipelined Memory Controller”, filed Jul. 31, 1998, whichis hereby incorporated by reference in its entirety. This application isalso related to U.S. patent application Ser. No. 09/127,207, titled“Pipelined Memory Controller,” filed Jul. 31, 1998, and having attorneyDocket Number MTIPAT.020A and U.S. patent application Ser. No. ______,titled, “Pipelined Memory Controller,” filed Jul. 18, 2001, and havingattorney Docket Number MTIPAT.020C1.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to memory controllers and moreparticularly to a method for processing memory requests in a memorycontroller.

[0004] 2. Description of the Related Technology

[0005] A computer system relies on memory to store instructions and datathat are processed by a computer system processor. Breathtaking advanceshave been made in both the storage capacity and speed of computer memorydevices. However, the speed increases of memory devices have not beenable to keep pace with the speed increases achieved with currentmicroprocessors. As a result, the speed of current computer systems islimited by the speed in which the data instructions can be accessed fromthe memory of the computer system.

[0006] The typical memory contains an array of memory cells connected toeach other by row and column lines. Each memory cell stores a single bitand is accessed by a memory address that includes a row address thatindexes a row of the memory array and a column address that indexes acolumn of the memory array. Accordingly, each memory address points tothe memory cell at the intersection of the row specified by the rowaddress and the column specified by the column address.

[0007] In a typical computer system, the system processor communicateswith the computer memory via a processor bus and a memory controller.For example, a central processing unit (CPU) issues a command and anaddress which are received and translated by the memory controller. Thememory controller, in turn, applies appropriate command signals and rowand column addresses to the memory device. Examples of such commandsinclude a row address strobe (RAS), column address strobe (CAS), writeenable (WE), and possibly a clock signal (CLK). In response to thecommands and addresses, data is transferred between the CPU and thememory device.

[0008] The memory device typically includes a dynamic random accessmemory (DRAM) module such as a single in-line memory module (SIMM) or adual in-line memory module (DIMM). The memory module typically includesone or more banks of memory chips connected in parallel such that eachmemory bank stores one word of data per memory address.

[0009] In an attempt to decrease memory access time, an even faster formof memory, referred to as synchronous DRAM (SDRAM), was created. SDRAMtransfers data with the use of a clock signal. In contrast, typical DRAMdevices are asynchronous because they do not require a clock inputsignal. The memory controller for synchronous devices receives thesystem clock signal and operates as a synchronous interface with the CPUso that data is exchanged with the CPU at appropriate edges of the clocksignal.

[0010] SDRAMs offer substantial advances in DRAM operating performance,including the ability to synchronously burst data at a high data ratewith automatic column-address generation, the ability to interleavebetween internal data banks in order to hide precharged time, and thecapability to change column in addresses on each clock cycle during aburst access.

[0011] Typically SDRAMs are configured to include a pipeline. Pipeliningrefers to the interlinking or overlapping of input and output data andaddresses of consecutive bus cycles. Pipelining increases the throughputof memory transactions. With this pipelined architecture SDRAMs canaccept a new column address on every clock cycle.

[0012] As the speed of memory devices such as the SDRAM increases, otherbottlenecks arise within computer systems. For example, as SDRAM devicesare operated at faster clock rates, the memory controllers to which theyare coupled often cannot exchange data between the CPU and the memorydevice quickly enough. Therefore, manufacturers have found that thememory controller itself needs to be pipelined.

[0013] In view of the above, it is apparent that manufacturers are inneed of an efficient pipelined memory controller to facilitate thecommunication of the memory requests to the memory devices.

SUMMARY OF THE INVENTION

[0014] One aspect of the invention comprises a method of processing atleast one memory request. The method comprises processing the at leastone memory request in a plurality of stages. In a first stage, the atleast one memory request is received from a memory requester and storedin a request buffer. In a second stage the at least one memory requestis decoded. In a third stage the at least one memory request is sent toa memory module. During the act of processing, each of the first, secondand third stages are executed in parallel, and wherein the act ofprocessing includes the act of storing the first and second memoryrequests in at least one rotational register contained in the requestbuffer.

[0015] Another aspect of the invention includes a method of handlingmultiple memory requests. The method includes receiving in a memorycontroller at least two memory requests for digital information for datathat resides a memory and concurrently processing the two memoryrequests.

[0016] Yet another aspect of the invention comprises a system comprisingat least one memory and a pipelined memory controller. The memorycontroller is capable of operable controlling the at least one memoryand receiving memory requests with respect to the memory. The pipelinedmemory controller is capable of processing the memory requests in aplurality of concurrently processed pipelined stages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block diagram of one embodiment of a computer systemaccording to the invention.

[0018]FIG. 2 is a block diagram of the pipeline modules of the memorycontroller shown in FIG. 1.

[0019]FIG. 3 is a block diagram illustrating some of the signal linesused by the state machine shown in FIG. 2 to update the pointers used bythe pipeline modules.

[0020]FIG. 4 is a flow diagram illustrating the memory receiving processof the request buffer shown in FIGS. 2 and 3.

[0021]FIG. 5 is a timing diagram of the pipeline components of thememory controller shown in FIG. 1.

[0022]FIG. 6 is a flow diagram illustrating the pipeline process for thestate machine shown in FIG. 3.

[0023]FIG. 7 is a flow diagram illustrating the data transfer process ofthe data transfer module shown in FIGS. 2 and 3.

[0024]FIG. 8 is a timing diagram of the pipeline components of thememory controller shown in FIG. 1 for two memory requests.

[0025]FIG. 9 is a flow diagram illustrating the pipeline process of thememory controller for the timing diagram shown in FIG. 5.

[0026]FIG. 10 is a state diagram illustrating the pipeline process forthe state machine shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following detailed description is directed to certainspecific embodiments of the invention. However, the invention can beembodied in a multitude of different ways as defined and covered by theclaims. In this description, reference is made to the drawings whereinlike parts are designated with like numerals throughout.

System Overview

[0028]FIG. 1 illustrates a computer system 10 for processing memoryrequests for access to a memory module 12 with two memory banks 14, 16.The memory module 12 can be any of numerous types of memory modules,such as a double in-line memory module (DIMM). The memory module 12includes a set of SDRAM memory chips structured to store data andcomputer instructions. The memory module 12 can include memory otherthan SDRAM, such as static random access memory (SRAM) or asynchronousDRAM.

[0029] Of course, the memory module 12 can have more or less than thetwo memory banks 14, 16. The computer system 10 includes a memorycontroller 18 that controls the manner in which data is written to orread from the memory banks 14, 16 of the memory module 12. The memorycontroller 18 interfaces the memory module 12 with a computer systemprocessor 20 via a processor bus 22. As is typical, the computer system10 also includes an extension bus, such as a PCI bus 24, that is coupledto the processor bus 22 by a bus agent 26. Coupled to the extension bus24 is an input device 28, including such peripherals as a keyboard,mouse, or electronic pen/tablet, and a hard drive 30. The computersystem 10 also includes a video monitor 32 coupled to a video controller34. The video controller 31 is coupled to the memory controller 18 by anaccelerated graphics port (AGP) bus 33. The processor 20, input device28, hard drive 30, and video controller 34, each can be referred to as amemory requester because each can request access to the informationstored in the memory module 12.

[0030] In addition, the PCI bus 24 can also be referred to as a memoryrequester because the input device 28, hard drive 30, and any otherdevices coupled to the PCI bus 24 all require the PCI bus 24 to transfermemory requests to the memory module 12. In one embodiment, the PCI bus24 and the video controller 34 are coupled directly to the memorycontroller 18 to allow direct memory access to the memory module 12 bythe PCI bus 24 and the video monitor 32 and the video controller 34.Such direct memory access reduces the data that is carried by theprocessor bus 22, thereby reducing bottlenecks on the processor bus 22and increasing the speed of communication between the elements of thecomputer system 10. In particular the memory controller 18 includes aprocessor interface 36, coupled to the processor 20 via the processorbus 22; a PCI interface 38 coupled to the PCI bus 24; and a videointerface 40 coupled to the video controller 34. Each of the interfaces36, 38, 40 is responsive to respective read and write control signalsreceived from each of the interfaces 36, 38, 40 respective memoryrequesters 20, 24, 34. The memory controller 18 also includes a pipelinecontroller 42 which handles all of the read and write requests to thememory module 12.

[0031] Referring to FIG. 2, in conjunction with FIG. 1, some of thefeatures of the pipeline controller 42 are illustrated. The pipelinecontroller 42 has a request buffer 50. The request buffer 50 has asimple rotational set of request registers 52-58 that hold the address,type of transfer and count for each memory request. The number ofregisters in the request buffer 50 determines how many outstandingrequests may be sent to the memory controller 18. In one embodiment ofthe invention, four registers are used. However, the request buffer 50may comprise sets of three to five, two to seven, or one to nineregisters as well. The pipeline controller 42 also has a decode module60, a memory address module 62, a data transfer module 64, and a statemachine 66.

[0032] Referring to FIG. 3, it is seen that the decode module 60, thememory address module 62, and the data transfer module 64, all contain aunique pointer to the request buffer 50. The state machine 66 controlsthe pointer updates for decode module, the memory address module 62, andthe data transfer module 64. The function and operation of each of themodules 60-64 will be described in greater detail below.

Method of Operation

[0033] The pipeline controller 42 uses a pipelined approach to handlinga memory request from each of the memory requesters. Each stage in thepipeline is handled by one of the modules 60-64 in the pipelinecontroller 42. The pipelined approach provides for the simultaneousprocessing of multiple memory requests. In the embodiment of theinvention illustrated in FIG. 2, a three stage pipeline is shown. Eachstage of the pipeline is handled by one of the modules 60-64,respectively. In a first stage, the decode module 60 determines whetheror not the first memory request was a page hit or a miss, the type ofmemory that had been requested, which of the memory banks 14, 16 is tobe addressed, and the error correction or detection method to be used.In a second stage, the memory address module 62 sends the row andaddress information of the memory request to the memory. In a thirdstage, the data transfer module 64 handles the transmission of the datato or from the memory module 12. The decode module 60 and the memoryaddress module 62 may process the same memory request independently ofeach other. However, the data transfer module 64 needs to wait for theappropriate information to be transferred to the memory module 12 beforesending or receiving the data for a memory request.

[0034] It is noted that the number of stages in the state machine 66 maybe increased or reduced. For example, a larger pipeline may beimplemented by increasing the number steps. The decode stage can bedivided into multiple steps. The pipeline controller 42 can have apipelined stage for each of the major processes in the decode module 60,the memory address module 62 and the data transfer module 64. If thenumber of stages is increased, the amount of time for each stage isreduced. The state machine 66 will control which phase of a memory cycleis active for each of these phases. Additionally, the state machine 66will determine when each of the pipeline modules 60-64 pointers are tobe incremented.

[0035] Referring to FIG. 4, the receiving process for the request buffer50 is illustrated. The request buffer 50 holds and queues each of thememory requests for processing for later use by each of the pipelinemodules 60-64. Starting at a state 68, the memory controller 18 receivesa first request. Moving to a decision state 69, the memory controller 18determines whether the request buffer 50 is full. If the request buffer50 is full, the memory controller in a state 70 notifies the memoryrequestor that the request buffer is full. No further requests can bereceived until one of the memory requests in the request 50 is fullyprocessed. Otherwise, if the request buffer 50 is not full, in a state71, the memory controller 18 places the memory request in one of theregisters 52-58. Moving to a state 72, the memory controller 18increments the request pointer to reference the new memory request. Thememory controller 18 returns to the state 68 to process further memoryrequests.

[0036] Now referring to FIG. 5, the process by which the decode module60 performs the initial processing for a memory request is described.Starting at a state 73, the state machine 66 assigns the decode pointerto reference the proper memory request in the request buffer 50. Then,in state 74, the decode module 60 determines whether or not the memoryrequest is a page hit or a miss. If the memory request is a hit, thememory reference is of a same type as a previous memory reference.Therefore, no additional decoding is performed and the process flowreturns to the state 73. However, if the memory request is a miss, theprocess proceeds to a state 76. At the state 76, the address of thememory request is compared to a set of bank address registers todetermine a bank number for the appropriate memory bank. The bank numberis used to reference a plurality of bank descriptor registers.Proceeding to a state 77, the bank desriptor registers are used todetermine the memory type of the memory request and the type of errorcorrection that are to be applied upon accessing the memory. The decodemodule 60 can be programmed to use any of the error correction ordetection methods that are known in the art. The process flow thenreturns to the state 73 to perform additional decoding. The operation ofthe page hit/miss determination, the bank address comparison, and thebank descriptor lookup may take one or more clock cycles depending onthe clock speed of the computer system 10.

[0037] The results of this processing are then saved for use by theother modules 62-64. Upon completion of the decode processing, the statemachine 66 increments the decode pointer to the request buffer 50. Thedecode module 60 now processes the next memory request, if any, in therequest buffer 50.

[0038] Referring to FIG. 6, the states of the second stage of thepipeline are described. The second stage of the pipeline processinvolves memory addressing. Similar to the decode pointer in the decodemodule 60, the memory address module 62 has an addressing pointerreferencing a memory request in the request buffer 50. Starting at astate 80, the state machine 66 updates the addressing pointer. Theaddressing pointer indicates to the memory address module 62 whichmemory request needs to be processed. Next, in state 82, the memoryaddress module 62 asserts the row address strobe (RAS) and the columnaddress strobe (CAS) and sends to the memory module 12 the row andcolumn address, respectively, for the address referenced by theaddressing pointer. Once the memory address module 62 is complete, theaddressing pointer is updated in state 84 so that it points to the nextmemory request.

[0039]FIG. 7 illustrates the third stage of the pipeline. The thirdstage is the data phase which is handled by the data transfer module 64.The data transfer module 64 has a data pointer which references thememory request in the request buffer 50 which is to be processed. Instate 86, the data pointer is updated by the state machine 66. The datatransfer module 64 then transfers the data to the memory module 12 instate 88. The amount of buffering in the memory controller 18 determineshow long this stage is active due to CAS latency. CAS latency is thedelay, in clock cycles, between the registration of a request commandwith a memory device and the availability of the first piece of outputdata. When the data transfer is complete, the data phase pointer isincremented in state 90 by the state machine 66.

[0040] Referring to FIGS. 8 and 9, the processing steps for two memoryrequests are illustrated. FIG. 8 is a timing diagram illustrating thesignificant portions of the memory controller 18, and FIG. 9 is a highlevel flow diagram showing the processing performed by the memorycontroller 18 for these requests.

[0041] Starting at a state 120 (FIG. 9), a first memory request isreceived by the memory controller 18 (clock cycle 100). The memorycontroller 18 places the first memory request in the request buffer 50.Since the first memory request is the only request in the queue, thememory controller 18 performs the decode and the address functionssimultaneously. The decode process is described in greater detail inreference to states 73-77; however in summation, the decode module 60determines whether or not the first memory request was a page hit or amiss, the type of memory that had been requested, which of the memorybanks 14, 16 is to be addressed, and the error correction or detectionmethod to be used. The memory address module 62 starts sending theaddress for the first memory request to the memory module 12. The memoryaddress module 62 asserts the row address strobe and sends the rowaddress to the memory module 12 across an address line.

[0042] Next, at a state 122, a second memory request is received (clockcycle 102). The state machine 66 updates the pointer in the decodemodule 60 to reference the second memory request. Moving to a state 124,the decode module 60 starts processing the second memory request (clockcycle 104). Additionally, the memory address module 62 asserts thecolumn address strobe and sends the appropriate address for the columnfor the first memory address.

[0043] Continuing to a state 126, the state machine 66 updates thepointer in the memory address module 62 to reference the second memoryrequest (clock cycle 106). The memory address module 62 during clockcycles 108-112 sends the memory module 12 the row and column informationfor the second memory request.

[0044] Finally, in state 128, the requested data is received or sent tothe memory module 12 (clock cycles 110 and beyond). It is noted thatwhile data is being received from the data module, the memory controller18 can decode and address subsequent memory requests.

[0045]FIG. 10 is a flow diagram illustrating the pipeline process forthe state machine 66. Starting at a state 150, the state machine 66 isin an idle state waiting for a memory request. After receiving a memoryrequest, the state machine 66 moves to a state 152 to send the rowaddress to the memory module 12 of the memory referenced by the memoryrequest. Next, at a decision state 154, the state machine 66 requeststhe memory module 12 to perform a column access. If the memory referencewas not in the memory banks 14, 16 of the memory module 12, the statemachine 66 returns to state 152 to process further memory requests, ifany. If the memory reference was in the memory banks 14, 16, the statemachine 66 proceeds to a decision state 156. In this state, an activecommand may be used to open or activate a row in the memory banks 14, 16for subsequent memory accesses. At decision state 156, if an activecommand was initiated, the state machine 66 determines whether thememory request was for burst access or whether there is a pipelinerequest for a memory cell in the same row as the previous request. Ifthe memory request was for a burst access, the state machine 66 moves toa state 158 and waits for the data. On the other hand, if it isdetermined in state 156 that a row in the memory banks 14, 16 was madeactive, in the decision state 156 the state machine 66 processes all ofthe memory requests for the open row. Moving from the state 156, thestate machine 66 moves to state 160 and activates the precharge commandto deactivate any active rows in the memory module 12.

[0046] As is witnessed by the foregoing discussion, the pipeline of theinvention increases memory throughput if several memory accesses areexecuted, one after the other. Based upon the foregoing discussion, itwill be appreciated that the invention greatly reduces the number ofmemory access delays by pipelining the memory requests in the memorybuffer 50. The invention allows three memory operations to be acted onconcurrently. The invention provides the decode module 60, the memoryaddress module 62, and the data transfer module 64 which can eachperform the partial processing of a memory request during the same clockcycle. In a non-pipelined memory controller, each memory request waitsfor the completion of the previous memory request before beingprocessed. However, using the modules 60-64, a second memory request canbe processed before the completion of the processing of a first memoryrequest. For example, once the decode module 60 has finished decodingthe first memory request, the decode module 60 can start the decodeprocess for the second memory request.

[0047] More significantly, the pipeline for the memory controller 18 issimply and cheaply implemented by using a set of rotational registers52-58. The registers 52-58 are easily indexed by stages in a pipelinethrough the use of pointers. The state machine 66 simply updates thepointers for each of the modules 60-64 to step through each of thestages in the pipeline.

[0048] While the above detailed description has shown, described, andpointed out fundamental novel features of the invention as applied tovarious embodiments, it will be understood that various omissions andsubstitutions and changes in the form and details of the systemillustrated may be made by those skilled in the art, without departingfrom the intent of the invention. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A method of processing at least one memoryrequest, the method comprising: processing the at least one memoryrequest in a plurality of stages, wherein in a first stage, the at leastone memory request is received from a memory requester and stored in arequest buffer, wherein in a second stage the at least one memoryrequest is decoded, wherein in a third stage the at least one memoryrequest is sent to a memory module, wherein during the act ofprocessing, each of the first, second and third stages are executed inparallel, and wherein the act of processing includes the act of storingthe first and second memory requests in at least one rotational registercontained in the request buffer.
 2. The method as defined in claim 1,wherein during the act of processing, each of the first, second andthird stages are executed during an identical number of clock cycles. 3.The method as defined in claim 1, wherein during the act of processing,each of the first, second and third stages are executed independently.4. The method as defined in claim 1, further comprising the act ofassigning a decode pointer, an address pointer, and a data pointer toreference the register of the memory request that is being processed byan decode module, an address module and a data module, respectively. 5.The method as defined in claim 1, wherein the act of processing furtherincludes the act of determining what type of memory is being requestedby the memory request.
 6. The method as defined in claim 1, wherein theact of processing further comprises the act of determining which databank of the memory module contains the data for the memory request. 7.The method as defined in claim 1, wherein the act of processing furthercomprises the act of determining an error correction method.
 8. Themethod as defined in claim 1, wherein the act of processing furthercomprises the act of sending the row and column address of the memoryrequest to the memory module.
 9. A method of manufacturing a pipelinedmemory controller, comprising: connecting a request queue having aplurality of registers to a state machine; and coupling a data transfermodule, a decode module and a memory address module to the statemachine, each of the modules having a pointer which is configured topoint to a memory request in the request queue.
 10. A method ofpipelining memory requests in a memory controller, comprising: storing amemory request in a first register; pointing a request pointer to thefirst register; pointing a decode pointer to the first register;pointing an address pointer to the first register; decoding the addressof the memory request in the first register; sending the address of thememory request to a memory module; storing a second memory request in asecond register; updating the request pointer to reference the secondmemory request; updating the decode pointer to reference the secondmemory request; updating the address pointer to reference the secondmemory request; decoding the address in the second memory address;incrementing the decode and address pointers; sending the address of thememory request to a memory module; and transferring the data requestedby the first and second memory requests from the memory module to a datatransfer module.
 11. The method as defined in claim 10, furthercomprising the act of updating the decode pointer, the address pointerand the data pointer by a state machine at the beginning of each stagein a pipeline.
 12. The method as defined in claim 10, wherein the act oftransferring the data further includes the act of supplying a clocksignal to a memory module.
 13. The method as defined in claim 10,wherein each of the acts of storing further includes the act of storingthe first and second memory requests in at least one rotationalregister.
 14. The method as defined in claim 12, wherein each of theacts of decoding further includes the act of determining what type ofmemory is being requested by the memory request.
 15. The method asdefined in claim 12, wherein each of the acts of decoding furtherincludes the act of determining which data bank of the memory modulecontains the data for the memory request.
 16. The method as defined inclaim 12, wherein each of the acts of decoding further includes the actof determining an error correction method.
 17. A method of handling atleast one memory request, comprising: receiving at least one memoryrequest for digital information from a memory requester, the memoryrequest being received by a memory controller that processes the memoryrequest in a plurality of pipelined stages; in one of the pipelinedstages, decoding the at least one memory request; in one of thepipelined stages, sending the at least one memory request to a memorymodule; and in one of the pipelined stages, communicating the data fromthe at least one memory request from the memory module to the memorycontroller.
 18. A method of handling multiple memory requests, themethod comprising: receiving in a memory controller at least two memoryrequests for data that resides in a memory; and concurrently processingthe two memory requests.
 19. The method of claim 18, additionallycomprising storing the at least two memory requests in a request buffer.20. The method of claim 18, wherein the request buffer includes aplurality of registers and wherein the method further comprisesassigning a decode pointer, an address pointer, and a data pointer toreference a register in the request buffer.
 21. A system comprising: atleast one memory; and a pipelined memory controller capable of operablycontrolling the at least one memory, the pipelined memory controllercapable of receiving memory requests with respect to the memory, andwherein the pipelined memory controller is capable of processing thememory requests in a plurality of concurrently processed pipelinedstages.
 22. The method of claim 21, additionally comprising storing eachof the memory requests in a request queue.
 23. The system of claim 22,additionally comprising a state machine for moving the memory requeststhrough the pipelined stages.
 24. A computer system, comprising: amemory module capable of storing digital information; and a memorycontroller, connected to the memory module, comprising: a request bufferfor receiving and storing multiple memory requests, a decode modulehaving a pointer to the request buffer, an addressing module having apointer to the request buffer, and a state machine capable of updatingthe pointer in the decode module and the addressing module, wherein thedecode module, the decode module and the addressing module operateconcurrently for at least selected time intervals.
 25. A computersystem, comprising: a memory module capable of storing digitalinformation; and a memory controller, connected to the memory module,comprising: a request buffer for receiving and storing multiple memoryrequests, a decode module having a pointer to the request buffer, anaddressing module having a pointer to the request buffer, and whereinthe decode module and the addressing module operate concurrently for atleast selected time intervals.
 26. The computer system of claim 25,additionally comprising a state machine which is capable of updating thepointer in the addressing module and the decode module.
 27. A system forprocessing at least one memory request, the system comprising: means forprocessing the at least one memory request in a plurality of stages,wherein in a first stage, the at least one memory request is receivedfrom a memory requester and stored in a request buffer, wherein in asecond stage the at least one memory request is decoded, wherein in athird stage the at least one memory request is sent to a memory module,wherein, each of the first, second and third stages are executed inparallel, and wherein the act of processing includes the act of storingthe first and second memory requests in at least one rotational registercontained in the request buffer.